1. Field of the Invention
This invention generally relates to an ultrasonic gas/liquid detection system to detect the presence/absence of liquid/gas in liquid pumps in general, and more particularly in glandless motor pumps which are in a steam generator loop of a fossil or nuclear steam generator.
2. Description of the Prior Art
Power generating plants use several electrical motors of various sizes for various purposes, the motors having electrical windings which need cooling for dissipation of heat generated in the motor windings. Invariably, motor windings in ordinary commercial applications are air cooled by a suction draft induced by fan means provided on the rotor of the motor. There are, however, some applications where the windings are totally enclosed or hermetically sealed, wherein a provision for heat dissipation must be made by a cooling fluid which is force-circulated in contact with "wet windings" to remove wet winding heat from the windings. In applications where an electric motor operates in association with a directly coupled pump to drive the pump, it is not uncommon to utilize the fluid driven by the pump to cool the inside of the motor to remove the heat generated by the motor windings. Expediently, in such instances, the fluid driven by the pump is also utilized for purposes of lubrication of the motor bearings; this is made possible by a construction known in the art as glandless motor pumps comprising (a) canned motors where the stator windings may not be cooled by the pump fluid, and (b) wet winding units where the motor stator winding is cooled by the pump fluid.
The use of canned motor and wet winding units as boiler circulating pump motors is not uncommon in steam cycle electrical generating plants. Reliability of the glandless motor pumps in electrical generating plants is a very significant requirement in power generating plants; it is of paramount importance especially in nuclear power plants wherein failure of a glandless motor pump can occur because of overheating caused by lack of cooling fluid for cooling the windings and the bearings. A lack-of-fluid situation is a very deleterious occurrence in glandless motor pumps whenever air or other gas is trapped inside the canned motor whereby the pump motor cavity is not vented. Accordingly, positive and timely detection of a lack of fluid situation is a very important requirement in glandless motor pumps especially where used for boiler fluid circulation. It is needless to emphasize that an improper and untimely occurrence of a lack of fluid situation in glandless motor pumps could result in damage to the power plant and harm to the operating personnel who might as a result be exposed to catastrophic consequences.
As additional structural features, these wet winding and canned motor units use no shaft seals or packing glands. Stainless steel alloys are used for important elements such as the rotor of the motor, the motor stator "can" and the pump labyrinth.
Considering the criticality of positively ascertaining if indeed a glandless motor pump has any air/gas which could potentially destroy the motor pump and more, certain kinds of detectors have been used hitherto. Of such detectors , the most notable ones have been the gas detectors known as Westinghouse Detectaires.RTM.. The "Detectaires" have been generally capable of detecting the presence/absence of gas/air with a fair degree of certainty; notwithstanding, the response time of such detectors has been rather unduly long, with the result that, often, the long delay before detection has already initiated damage or other problems. Delays of over 30 seconds with known detectors such as the Westinghouse Detectaires have been reported, resulting in total irreversible failures of bearings and cans before corrective action could be initiated. Moreover, the known Westinghouse Detectaire systems are relatively expensive and cost in the range of over U.S. $10,000 per unit. There has been a need therefore for a relatively inexpensive and fast acting detector for gas/air trapped in glandless motor pumps, the detector having such response as would obviate the disadvantages of the prior art arrangements.
U.S. Pat. No. 4,341,116 to Bilstad et al. teaches a system for detecting the absence of a liquid in a liquid chamber. An ultrasonic signal is transmitted through the liquid chamber, and, a signal received through the liquid chamber is fed through a control circuit to a comparator. A difference between a reference voltage in the comparator and the control circuit output voltage indicates ehether or not there is liquid in the liquid chamber. In this arrangement there is no teaching of any glandless motor pump wherein the fluid per se which is pumped by the motor pump unit is used for cooling the motor windings and bearings. Also, there is no additional means in this prior art arrangement to cool the pumped fluid or to regulate the temperature of the cooling fluid within the liquid chamber in any manner.
U.S. Pat. No. 3,407,398 to R. J. Stearn teaches a liquid presence detector which is in the form of a probe. The probe comprises an electro-acoustical transducer at one end of a body and a reflector plate which is located spaced from the other end of the body. The presence of liquid in the space between the transducer and the other end is detected by the reflections emanating from the reflector plate. In this arrangement, the probe has to be necessarily immersed or lowered in the liquid, before the liquid presence can be detected.
U.S. Pat. No. 3,744,301 to Arave teaches a detector arrangement to indicate liquid level in a gasliquid mixture. An ultrasonic burst is propagated in a wire, and, the attenuation of the ultrasonic burst is measured to determine the void fraction in the gas liquid mixture. When there exists a layer of gas over a liquid, the detector arrangement can be used as a liquid level indicator. There is no teaching in this prior art reference of any glandless motor pump wherein the liquid per se which is pumped by the motor pump is used for cooling the motor windings and the bearings.
In U.S. Pat. No. 4,144,517 to Baumoel, an arrangement is taught wherein the presence or absence of a liquid at a particular location of a tank or a pipe is sensed through the wall of a container by a single ultrasonic transducer. The transducer is mounted secured to the container wall, and projects a longitudinal sonic beam in the container wall. The presence of liquid or gas in the container causes different degrees of interface ultrasonic reflections. The reflections are compared with known decay rates of the reflections to ascertain if the container was empty or full at the time of measurement. Output circuits are provided to furnish "empty" or "full" signals or other signals in response to fluid level measurements. However, in this prior art arrangement, use is made of a reference signal decay time, with which is compared a measured decay time for the signal to attenuate from a first reference level to a second level which is 20% of the first reference level. There is also a question in this cited art as to how well the transducer is coupled with the container, and, to accommodate for improper coupling, compensatory measures are discussed. Notwithstanding, this prior art arrangement is likely to pose a delay in obtaining a final signal, which delay might be undesirable for wet winding units of boiler (primary water) circulating pumps.